1. Field of the Invention
The invention relates generally to nitride etch baths and more particularly to a nitride etch bath wherein the evaporation rate of the water vapor emitted from the bath is controlled by a condensing lid and collar.
2. Description of the Prior Art
In the semiconductor industry, boiling acid in a bath is utilized in the manufacturing process to etch away exposed areas of a wafer. A problem exists in that acid has a high concentration of water and at high temperatures (140-180).degree. C., the water evaporates at a rapid rate because water boils at 100.degree. C. If the water in the acid evaporates, the chemical concentration in the bath varies changing the pH of the acid and in the extreme results in straight acid. If the solution becomes straight acid, a condition known as "heavy acid" results which accelerates the etching process. Under these conditions, an operator cannot control the etch rate in the bath. A solution to this problem is to control the evaporation rate of the water in the acid.
Attempts to solve this problem in the prior art included adapting a collar to fit over the top of a rectangular acid bath. The collar was constructed with an inlet to and an outlet from a series of quartz coils. The quartz coils carried a condensing medium and the quartz coils were suspended adjacent to the inner walls of the acid bath. A quartz lid rested on top of the collar and as the vapor fumes rose from the bottom of the acid bath the quartz lid reflected the fumes towards the collar. The collar then condensed to a liquid state those fumes that contacted the collar. A major problem with this solution was that if the water pressure changed, as when the system valves were not closed when the system was not utilized, the quartz collar would shatter due to stress caused by a sudden increase in water pressure. The quartz lids, being very heavy, were also subject to breakage and were expensive to replace. The quartz lid also leaked acid vapor droplets when removed from the acid bath making removal a hazard.
Another attempt to prevent the loss of water vapor from the acid bath included the fitting of the walls of the acid bath with teflon coils. As in the prior art, a heavy dense lid was placed over the acid bath. The heavy dense lid performed in the same manner as the quartz lid with all the previously described problems. The dense lid reflected the acid vapor fumes at the top of the bath and the teflon coils mounted on the bath walls performed the condensing function. As before, the side mounted coils were not efficient and the teflon on the condensing coils proved to be a thermal insulator further reducing the efficiency. A third attempt to prevent the loss of water vapor from the acid bath included mounting U-shaped condensing coils on the bottom of a quartz lid mounted on a quartz acid bath.
Another problem existed in sensing the liquid level in the acid bath. If the level dropped, the heater element could destroy itself. Attempts to solve this liquid level sensing problem included using a pressure switch within the tank. Increase in tank temperature as the liquid level dropped would not cause the pressure switch to operate properly because the air about the thermocouple operated pressure switch carried off the heat. Any instrument within the tank caused contamination to the acid bath and ultimate destruction to the instrument due to acid or acid fumes. Also, optical sensors mounted laterally to the acid bath failed to operate due to exposure to the acid fumes and due to refraction of the transmitted light wave because of multiple layers of the vessel. Although the condensation efficiency was improved, the previously described problems associated with quartz tops remained and the problem of controlling the water evaporation rate from the acid bath and sensing the liquid level still remains.